Barrel type locks usually have a rotatable barrel, to which the locking arm or hasp is fastened, and which rotates with the barrel, between locking and unlocking positions. A series of locking pins or tumblers hold the barrel securely against rotation when in the locked position. Insertion of a key displaces the tumblers or pins into a predetermined position, freeing the barrel for rotation. This type of lock is relatively easy to pick, and offers little real security against theft. Since the barrel is held against rotation by the tumblers, a skilled person can feel the position of each tumbler and can open the lock relatively easily, without a key.
Locks of this type may be used to secure a dwelling for example or any other premises, and also find major application in the locking of cabinets, vending machines, display windows, photocopying machines and the like.
Various attempts have been made to render such locks thief-proof. For example, it has been proposed to separate the locking arm from the barrel and make them independently moveable relative to one another, and leaving the barrel free to rotate, when the locking arm is locked. Some form of clutch is provided between the barrel and the locking arm or hasp, and insertion of the key displaces the locking pins or tumblers, and procures engagement of the clutch so that rotation of the barrel will then rotate the locking arm.
This principal of operation makes such a lock much more difficult to pick, since there is no way in which the thief can sense the movement of the tumblers while he is attempting to pick the lock. Examples of such freely rotatable barrel locks may be found in U.S. Pat. Nos. 474,519, 2,690,070, 3,330,141, and 2,483,365 and in French Pat. No. 1,333,207.
However, these earlier proposals have suffered from various disadvantages. Locks of this type are frequently required for use within a restricted space, and often cannot be more than an inch or an inch and a half in length. The design of such earlier types of locks has been such that they could not be manufactured within such a restricted dimension, and still produce an effective and reliable lock.
Other disadvantages have involved the difficulty of providing a sufficient number of combinations of tumbler positions, and the relatively unsatisfactory shape or design of the key, and the general engineering and design requirements of such locks in some cases made them difficult to machine in mass production.